18 Prof. W. King on Spirifer cuspldatus. 



At first I could only be certain about the existence of the 

 septum in its simple form. It was divided in the middle, or, 

 in other terms, composed of two portions, each projecting from 

 the inside of one of the dental plates, and separated from each 

 other by a considerable interspace (fig. 24). Finding it in a 

 similar condition in another specimen, I began to suspect that 

 the component parts always remained disunited, as prevails in 

 Spirifer mosquensis. In one specimen the division presented 

 some appearance of being an incomplete canal ; but, not being- 

 certain, I rubbed down the umbone of another specimen, two 

 inches in width, and thus exposed the septum, well developed 

 and enclosing the canal, as in fig. 25. I could have no doubt 

 whatever that in this case the septum was canaliculated ; but, 

 in order to place the matter beyond dispute, I gave the speci- 

 men a further rubbing, which, though resulting in the oblite- 

 ration of the circular form of the canal, confirmed the view I 

 had taken, by disclosing it as a narrow slit dividing the sep- 

 tum (fig. 26). I have obtained corresponding results by simi- 

 larly operating on some other species*. Spirifer Verneuilij 

 from Boulogne, has the septum ; but the canal is feebly indi- 

 cated, just as it appeared in the second rubbing of Professor 

 Harkness's specimen of Sp. cuspidatus. In Sp. grandicostatus^ 

 M^Coy, both are tolerably well displayed, the canal being 

 situated on the back of the septum ; the next rubbing entirely 

 removed the canaliferous septum, leaving nothing but the 

 dental plates extending right across the umbonal cavity (fig. 

 28). This was also the case in Sp. striatus and some other 

 species : sometimes the outer half of the dental plates disap- 

 peared along with the septum. Sp. crassusj De Koninck, 

 showed the apophysis somewhat obscurely at first; but on 

 further rubbing, it became quite distinct (fig. 29). 



In all the species last mentioned the space between the 

 dental plates above the septum is simply filled with crystalline 

 or amorphous mineral matter. In some others the same space 

 contains a shelly deposit, which is particularly the case in the 

 Permian Sp. alatusf. At first the latter circumstance seems 

 to favour the idea that the arch is simply an excavation in the 

 shell-substance of the pertaining valve. But this cannot be 

 the case ; for, as will be seen in fig. 9, the dental plates (&, Z>), 



* The canaliferous septum is confined to the incurved or apical portion 

 of the umbone, on which account it is with difficulty detected, and only 

 by a close and frequent examination of the dififerent surfaces obtained by 

 rubbing. The canal is not often distinctly seen, 



t The arch in this species does not always contain a shelly deposit, as 

 casts before me, from Humbleton "Hill, show that the inside of it has 

 been quite divostod of any extraneous infilling'. 



